1. Field of the Invention
The present invention relates to a fuel-injection apparatus adapted to direct-injection engines.
2. Description of the Prior Art
Of the various fuel-injection apparatus that have been conventionally developed, for example, the pressure-accumulated, fuel-injection apparatus is widely used, in which fuel stored in the common rail under pressure is injected into the combustion chambers by the closure and open of the solenoid-operated valves provided on the heads of the injectors. A fuel-injection apparatus, for example, shown in FIG. 5, has a needle valve 59 that is movable in a nozzle body 57 for a reciprocating manner to open and close discharge orifices 58 at the tip of the nozzle body 57, the needle valve 59 being constantly urged by the action of a closing spring 60 to close the discharge orifices 58.
The nozzle body 57 is provided therein with a fuel passage 62 for allowing the fuel fed under high pressure from the common rail to flow into a fuel sac 61, a fuel passage 64 for allowing the fuel fed under high pressure from the common rail to flow into a balance chamber 63, a fuel passage 67 for communicating the balance chamber 63 with a space 66 containing therein a solenoid-operated valve 65 to open and close the fuel passage 67, a fuel-leak passage 70 for communicating the space 66 with an intermediate chamber 69 surrounding around a slender section 68 of the needle valve 59, and a fuel discharge passage 71 for communicating the space 66 to a fuel reservoir.
The needle valve 59 has upper and lower sections arranged on the axially opposing end of the spender section 68, the upper section being made greater in diameter, compared with the lower section. The needle valve 59 is subjected to the hydraulic pressure, or fuel pressure, of the high-pressure fuel forced into the fuel sac 61, which acts so as to open the discharge orifice 58, and at the same time subjected to the resultant force of the urging force of the closing spring 60 and the fuel pressure of high-pressure fuel in the balance chamber 63, which acts so as to close the discharge orifice 58. As the intermediate chamber 69 is exposed to the low fuel pressure acting through the fuel-leak passage 70, the high-pressure fuel in the fuel sac 61 and balance chamber 63 may leak out through a clearance between the confronting needle valve 59 and the nozzle body 57. The fuel leaked out in the intermediate chamber 69 is collected in the reservoir through the fuel-leak passage 70, intermediate chamber 66 and fuel discharge passage 71.
When energizing the solenoid-operated valve 65, a valve body 72 of the solenoid-operated valve 65 is attracted to an electromagnet against an elastic force of a return spring 73. At this event, the fuel passage 67 is open to the space 66 whereby the balance chamber 63 is relieved through the fuel passage 67, resulting in the reduction in the fuel pressure therein. The force of fuel pressure acting on the fuel sac 61 is designed greater than the resultant force of the force of fuel pressure acting on the balance chamber 63 with the spring force and, therefore, the needle valve 59 moves upwards, resulting in opening the discharge orifice 58 as shown in FIG. 5. In contrast, when deenergizing the solenoid-operated valve 65, the valve body 72 moves downwards by the action of the return spring 73 to thereby close the space 66 at the fuel passage 67. As a result, the resultant force of the elastic force of the spring 60 with the force of fuel pressure restored in the balance chamber 63 becomes greater than the force of fuel pressure acting in the fuel sac 61 to make the needle valve 59 move downwards thereby closing the discharge orifice 58.
On the fuel-injection apparatus as described above, however, the fuel leaks constantly out from the intermediate chamber 69 through the fuel passage 67, namely, the fuel is constantly under the static leakage, in addition to that the fuel leaks out from the balance chamber 63 to the space 66 through the fuel passage 67 at every actuation of the solenoid-operated valve 65. Moreover, as seen from FIG. 6, the amount of static leakage of fuel increases with the increase of the common rail pressure. That is to say, as the pressure in the intermediate chamber 69 is constantly under the low pressure or the atmospheric pressure, the increase of the fuel pressure results in increasing the amount of the leakage of fuel from the fuel sac 61 and balance chamber 63 to the intermediate chamber 69 through the clearance between the confronting needle valve 59 and nozzle body 57.
To cope with the problem as described just above, a fuel-injection apparatus was developed, which is disclosed in Japanese Patent Laid-Open No.77924/1998. The fuel-injection apparatus, as apparent from FIG. 7, has for its object to make sealing with the use of fuel pressure and is comprised of a body having fuel-discharge orifice equivalent to the reference number 58, a needle valve 75 movable in a space 74 in the body in a reciprocating manner so as to open and close at its one axial end the fuel-discharge orifice, a balance chamber 76 in which the needle valve 75 is exposed at its axially opposite end serving as a pressure-supporting face to control the amount of lift of the needle valve 75, a fuel-supply passage 77 for applying the fuel pressure to the balance chamber 76, a fuel-discharge passage for relief of the fuel pressure in the balance chamber 76, a valve 79 for opening and closing the fuel-discharge passage 78, and an actuator for operating the valve 79. The actuator-operated valve 79 is composed of a valve stem 80 extending through the fuel-discharge passage 78 into the balance chamber 76, and a valve face provide at the tip of the valve stem 80 so as to make a contact with a valve seat formed at the ingress of the fuel-discharge passage 78. The actuator-operated valve 79 is made of the valve stem 80 and valve body 81 that are formed integrally with each other.
When the actuator is not energized, a return spring 82 forces the valve stem 80 upwards through a spring shoe 83 while the valve face abuts against its associated valve seat and, therefore, the actuator-operated valve 79 closes the fuel-discharge passage 78. On this event, the high-pressure fuel from the common rail is applied to the fuel sac, shown at 61 in FIG. 5. The fuel in the sac forces the needle valve 75 to the direction of lift. Moreover, the fuel pressure applied to the balance chamber 76 through the fuel passage 77 acts on a pressure-support face 87. At this instant, the resultant force of the spring force of a diaphragm spring 88 and the force of fuel pressure acting on the pressure-support face 87 of the needle valve 75 exceeds the force of fuel pressure applied in the fuel sac, which is exerted on the needle valve 75 to open the discharge orifice, so that the needle valve 75 is held closed to stop the fuel injection out of the discharge orifice.
The instant the actuator is energized, the valve stem 80 is forced downwards in FIG. 7 against the compressed spring force of the return spring 82, moving the valve face of the valve body 81 off its seat, whereby the actuator-operated valve 79 opens the fuel-discharge passage 78. The fuel passage 77 has the effect of a kind of iris, which renders the flow of fuel in the fuel passage 77 smaller than that in the fuel-discharge passage 78. Therefore, opening the fuel-discharge passage 78 results in relieving the balance chamber 76 of the fuel pressure to the space 74. The instant the fuel pressure in the balance chamber 76 is relieved, the force to move the needle valve 75 towards opening overcomes the resultant force of the spring force of the diaphragm spring 88 and the fuel force acting on the pressure-support face 87 of the needle valve 75, which urges the needle valve 75 to the direction of closing. This raises the needle valve 75 off its seat whereby the fuel is injected out of the discharge orifice into the combustion chamber.
On deenergizing the actuator, the valve stem 80 is lifted up by the action of the return spring 82 to close the actuator-operated valve 79. The fuel pressure in the balance chamber 76 is restored by the fuel supply through fuel passage 77 to thereby force downwards the needle valve 75 to close the discharge orifice with the result that the fuel injection ceases. The restored fuel pressure in the balance chamber 76 acts on the valve body 81 and consequently urges, in addition to the force of the return spring 82, the valve face against the its seat. It will be understood that the higher the fuel pressure in the balance chamber is, the greater is the force opening the actuator-operated valve 79, which may be thus kept certainly against the fuel leakage.
In the meantime, the actuator-operated valve 79 for opening and closing the fuel-discharge passage 78 to the balance chamber 76 has been conventionally produced by forming integrally the valve stem 80 with the valve body 81. Such integral forming is preferred for producing the large-sized valves in view of mechanical strength and production cost. Nevertheless, as the actuator-operated valve 79 for the fuel-injection apparatus has the valve stem 80 and valve body 81, which are usually in the range of from about zero point several millimeters to at most several millimeters in their diameter, it becomes much more difficult to achieve the mechanical strength and finish accuracy of the actuator-operated valve 79, which are durable to the recent high-injection pressure.
In integral forming of the valve stem 80 with the valve body 81 of the valve 79, there has been a limit to form the valve stem 80 and valve body 81 slender with high accuracy and, therefore, it could not be helped to make them larger in diameter. For the reasons described above, it has been impossible to make the sufficiently smaller size for the part exposed to the pressure or the area of the pressure-support face of the actuator-operated valve 79. Increase in high fuel-injection pressure causes increase in thrust force exerted from the pressure-support face on valve opening and consequently the valve opening requires the valve-operating power increasing in proportion to the thrust force on valve opening. This inevitably has resulted in using the actuator relatively large in its size, which has caused a major problem in incorporation into the engine.